A practical way to double the bit carrying capacity of an existing unidirectional fiber optic communication link is by the use of optical circulators. An optical circulator is a passive, nonreciprocal device which permits full duplex communication on a single fiber optic link. Thus, a typical fiber optic communication link operating on two fibers can be quickly and economically converted to a bidirectional, single fiber communication link by installing an optical circulator at each end of the link.
FIG. 1 illustrates in block diagram form the operation of an optical circulator. From a block diagram point of view, the optical circulator can be viewed as a three-port device, having ports 1, 2, and 3. Light that enters port 1 exits the optical circulator at port 2. However, light that enters the optical circulator at port 2 exits at port 3. The fact that an optical circulator treats light moving in different directions differently makes it a non-reciprocal device. Though various designs are possible, the most commonly used component which gives a bulk optics circulator its non-reciprocity is a Faraday rotator.
FIG. 2 illustrates, in block diagram form, how a pair of optical circulators can be used to provide simultaneous, bidirectional communication on a single fiber optic link. Optical circulators 10 and 12, each having ports 1, 2, and 3, are installed at opposite ends of fiber optic link 14. For each optical circulator 10 and 12, a communication transmitter is located at port 1, the fiber is connected to port 2, and a communication receiver is located at port 3. In this manner, light emitted from each transmitter is launched onto fiber link 14 from opposite ends in opposite directions. At the end of each respective path, optical circulators 10 and 12 separate incoming signals from outgoing signals, so that the transmitters and receivers do not interfere with each other.
One of the major advantages of optical circulators over more traditional 3 dB couplers is that the loss penalty is much lower. Using a 3 dB coupler at each end of a fiber link, there is a guaranteed insertion loss of at least 6 dB. For connections which operate near their detection limits, this additional 6 dB loss could make bidirectional communication unworkable.
In a real optical circulator, three important considerations are insertion loss, cross-talk, and coupling loss. Insertion loss is the difference in power between light launched into the optical circulator and the power that exits the device. Insertion loss is primarily due to absorption of light and to imperfect polarization separation.
Cross-talk in an optical circulator refers to the amount of power emitted at port 3 (to the receiver) from light entering at port 1 (from the transmitter). Cross-talk is represented in FIG. 1 by the dashed line from port 1 to port 3. Due to fiber losses, the near end transmitter of a fiber optic communication system is generating much higher power levels than the near end receiver would normally see from the far end transmitter. If cross-talk is too high, the coupling of power from transmitter 1 into receiver 1 in FIG. 2 will overpower the signal from distant transmitter 2 and make the optical circulator useless for telecommunications purposes.
The primary cause of cross-talk in optical circulators is back-reflection from the various optical elements in the device. Since the optical circulator `steers` the light rays depending on the direction of the rays, light originally from the near end transmitter but back-reflected from certain surfaces in the device and fiber are treated exactly as if these rays originally came from the far end transmitter. In addition to cross-talk within the optical circulator itself, reflections from the devices used to couple the optical circulator to the fiber can also cause cross-talk. Thus, otherwise useable connections or devices in a unidirectional fiber link could cause major problems if used in a bidirectional link. Therefore splices and connectors, as well as the internal components of the optical circulator, should have back-reflections minimized.
It is worth noting that reflections from the communication receiver itself do not cause a cross-talk problem since an optical circulator is in fact a four port device. The fourth port is not used in a bidirectional communication system, and therefore back-reflections from the receiver are propagated to port 4 where they are lost to free space or to an absorbing material.
Coupling losses arise from absorption, reflection, or stimulation of radiating modes in the fiber which may occur at a connection or splice between fiber strands, or between an optical fiber and an optical device. Like insertion losses and cross-talk, coupling losses, if not minimized, can make bidirectional fiber optic communication unworkable.
Prior art optical circulators are described in U.S. Pat. No. 4,650,289, issued to Kuwahara; U.S. Pat. No. 4,464,022, issued to Emkey; and in U.S. Pat. No. 4,859,014, issued to Schmitt et al. However, insertion loss and/or cross-talk in optical circulators made as described in these references are unacceptably high for many communications applications. Therefore, a need exists for an optical circulator having lower insertion loss and cross-talk than that found in present optical circulators. Such an optical circulator will preferably comprise entirely optical fiber which can be fusion spliced into a fiber optic communication system, thus avoiding coupling losses associated with traditional designs.